FIG. 6 shows a cooling device for a motor in the prior art. The cooling device includes a radial plate type impeller 2, which is supported by a rotary shaft 1a of a motor 1, and a fan cover 4, which encases the impeller 2 and which is formed from metal plates. Reference numbers 3, 5, and 6 respectively denote a vane, an air inlet, and an air outlet. In a cooling device having such a structure, the radial plate type impeller 2 is just encased in the fan cover 4. Thus, the air flow produced by the impeller 2 can only be expected to be directed toward the surface of the motor 1. Accordingly, application of the cooling device to a compact high-output motor results in a problem in which the fan capability causes insufficient cooling. Application of the cooling device to a high speed motor results in a problem in which the fan operation noise increases.
Various measures have been taken to solve the above problems. Japanese Laid-Open Patent Publication No. 11-289716 describes a cooling device in which an air flow passage of a motor cooling fan is diagonal to the rotary shaft. Additionally, the vanes have an outer diameter that is greater than the outer diameter of the housing of the motor.